Bootloader is code that is executed before any Operating
System starts to run. The concept of bootloaders is universal to
virtually all Operating systems that inculcates operating systems on
your PC, laptop, Smartphones, and other such devices. Bootloaders
basically package the instructions to boot operating system kernel and
most of them also have their own debugging or modification environment.
As the bootloader kicks off before any piece of software on your device,
it makes it extremely processor specific and every motherboard has it’s
own bootloader. This is one reason that all Android phones have
different Custom ROMS developed due to the high variance of processing
hardware present on the device. The iDevices running iOS somewhat share the same hardware specs and hence you don’t see much variance in their bootloaders.

Bootloader is usually locked on an Android device because it’s
an open source OS, still the manufacturers want you to stick to their
Android OS version specifically designed for the device. In order to
apply this concept, manufacturers lock the bootloader. With a locked
bootloader on Android devices, it is virtually impossible to flash a
Custom ROM and forced attempts void warranty as well as usually end up
in bricks. Therefore, the first step is to always unlock the bootloader.

What Does it Mean to Unlock the Bootloader?
Unlocking the bootloader is the first step towards gaining root
privileges. Some developers and enthusiasts unlock and root in order to
experiment with applications or with custom builds of Android. Needless
to say, you should not attempt to unlock the bootloader unless you know
what you are doing.

Hope you have Root your Sonny Ericsson Xperia with the tutorial I posted HERE, the next step to install Custom Rom to your phone is to unlock the Bootloader.Before unlocking bootloader you should check whether it is safe to unlock or not.

5. Then
you will get a message in which says "to switch off your device and
connect to PC pressing and holding back button in your device" and do
it.

6. The unlocking process will begin. Wait until Flashtool ask you to disconnect the cable off the phone, then turn on the device and connect the USB cable once again

7. Then flash tool says ”Successfully applied unlock Rebooting”

8. Then your bootloader is unlocked

To Check Whether Bootloader is Unlocked or Not

1. Run s1tool and click "identify" button on top right corner. (Don't connect your device)

2. Now switch off your device and connect to PC pressing and holding back button in your device.

3. Then under "PLEASE ATTACH TURNED OFF PHONE NOW" this line

we can see: RUNNING S1_EROM VER "r9A021" , in which "r" should be in small letter now, as indicate in the below screenshot.

4. This means you have successfully unlocked the bootloader!

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Mr Funsho, Thanks for your good works. Pls., that Etisalat BIS when topped it up with Z10 daily plan of N1000 and configured it is working on other devices but no more working on BB. Is there anything that can be done to be able to use it on BB. I was thinking it will be flexible to use on BB and other devices. Thanks once oahlL

You have a good question but why i ignore it since is dat it was not placed in d appropriate thread 4 others to benefit 4rm my reply.When, bb 10 daily plan of *599*3*2# automatically turns it into a normal data bundle and therefore, you may not be able to use it as a bis plan on your bb device. To use it again on your bb device just activate another bb daily plan by dialing *499*3*2# or Send COD to 399, costs N100 and it will be reactivated on your blackberry device.